Beth Carlton, a professor of epidemiology at the Colorado School of Public Health, said a digital approach to contract-tracing had the potential to be especially helpful for COVID because the virus has two tricks that often stymie more traditional public health efforts. First, it can be spread by people before they have symptoms or by people who never showed symptoms at all, making it difficult for those who are infected to avoid exposing others in public places. And, second, it can spread so rapidly that it can overwhelm manual contact-tracing efforts.